1. Field of the Invention
This invention relates to a navigational aid system and, more particularly, to a navigational aid system for a vessel which periodically corrects positional information stored in an addressable peripheral memory device of a microprocessor with satellite provided electronic positional information at predetermined intervals of distance traveled by the vessel such that current set and drift can be electronically determined onboard.
2. Discussion of the Related Art
U.S. Pat. No. 4,340,936 to Mounce discloses a navigational aid system for the navigation of a vessel. The Mounce system receives information from a number of sensors, and then processes the information by a microprocessor in order to operate a display that indicates various quantities germane to the operation of the vessel. The specific information inputs from the sensors are wind direction, wind speed, heading (as determined by a compass), hull speed (the velocity of the vessel through the water), and leeway angle (the angle between the direction of motion and the center line of the vessel).
The Mounce system also requires the manual input of certain data into the random access memory (RAM) of a microprocessor, and particularly the current set and drift in the portion of the body of water in which the vessel is to navigate, and the course and distance to a destination. The microprocessor uses the inputs to provide running information to aid in the operation of the vessel. It has been found that accurate information pertaining to current set and drift is difficult to obtain, however, because it changes with time and the location of the vessel. Fortunately, electronic systems are now available that provide highly accurate electronic positional information. A microprocessor aboard a vessel that utilizes such electronic information would eliminate the need for inserting the current set and drift manually. Thus, there exists a need for a self-sufficient navigational aid system that may be placed aboard a vessel and is capable of receiving and using external electronic positional information to determine current set and drift, thereby eliminating any need to manually input the same information. Such a resulting self-sufficient navigational aid system would constitute an improvement over the Mounce system.
Any navigational system which determines vessel position by a summation of continuously taken data is subject to accumulated errors. Consequently, the accuracy of the position of the vessel determined by such a navigational system deteriorates with time and distance from its starting position. The recently developed electronic systems provide absolute position to a high degree of accuracy and enable the periodic correction of the generally inaccurate position determined by the summation process.
To satisfactorily accomplish the determination of current set and drift and position correction, the electronic positional information system must provide: (1) position information of the highest possible accuracy; (2) a means for transferring that position information to the RAM of a microprocessor aboard a vessel; and (3) a rate of response that is compatible with the rate at which information is available so that a display aboard a vessel can adequately indicate the rate of change of the physical quantities.
The first electronic position information system that offered worldwide coverage is known today as Omega. Omega uses a number of very low frequency transmitters, located at various positions on the globe, that emit transmissions capable of being received practically anywhere on earth. The Omega system uses very precise timing to determine the difference in distance between two or more transmitters and the point of reception, and uses that distance to determine a fix. However, the Omega system as presently available is cumbersome and of insufficient accuracy to be used as a source of positional information for the purposes of this invention.
A second system known as LORAN is also presently in wide use. LORAN operates in a manner similar to Omega, but on a vastly smaller scale. Particularly, radio transmitters are precisely located, usually along a coast line, and their transmissions are accurately synchronized such that by measuring the phase difference of the signals from two or more transmitters, a position fix can be determined. Present LORAN receivers, under ideal conditions, are capable of providing the type of information required by the microprocessor navigation system of this invention. However, the accuracy of position fixing is highly variable and depends on the arrangement of the transmitters and their distances from the point of reception. Furthermore, LORAN is not available worldwide. Accordingly, the LORAN system is also inadequate for the purposes of this invention.
There are presently three known systems utilizing satellites that provide electronic positional information. The first system is known as Satnav and is already becoming obsolete. The Satnav system will eventually be replaced by another system known as the Global Positioning System (GPS) which will provide worldwide coverage when a full complement of satellites is finally in orbit. A somewhat similar system is being implemented by Russia. The GPS is the preferred system for use in this invention. Even though the full complement of satellites is not yet operational, worldwide coverage is available, although for less than 24 hours per day. Furthermore, GPS receivers are available at a moderate cost and provide information readily utilized by a microprocessor-based navigation system.
GPS receivers are available that provide combined numeric and alpha readouts of some of the quantities provided by the microprocessor-based system of this invention. But, the only actual measurement performed by the GPS receiver results in the determination of present position, which must be compared to a past position in order to calculate quantities such as speed and heading which can be read out. In other words, the determination of any information other than present position by the GPS is obtained indirectly by calculation using additional positional data, in contrast to the known microprocessor-based system which receives that same information directly from onboard sensors.
The proper navigation of a vessel, and especially a sailing vessel, requires a knowledge of: (1) the actual speed and direction in which the vessel is moving relative to the bottom; (2) the direction the vessel should move in to sail directly to a destination; and (3) the wind direction and speed.
The actual speed and direction in which the vessel is moving relative to the bottom in turn depend on the following variables: (a) the compass heading or direction assumed by the center line of the vessel relative to the earth, (b) the speed of the vessel through the water in the center-line direction; (c) the leeway or velocity of the vessel relative to a direction perpendicular to the center line; and (d) the current set and drift, which is the direction and speed of the movement of the water in the area surrounding the vessel. The microprocessor-based system of this invention considers all of the variables (a)-(d) in calculating the actual speed and direction of the vessel relative to the bottom. In addition, the system of this invention provides a readout of the variables (a)-(d) which greatly aids the operation of the vessel in certain situations. For example, a knowledge of leeway can assist in proper sail adjustment, and a readout of the speed of the vessel through the water aids in sail trimming. Although a readout of these variables by a GPS receiver is possible, the accuracy and rate of response are so linked together that an adequate accuracy is not obtainable along with an acceptable rate of response.
With respect to accuracy, the GPS has sufficient accuracy for the determination of current set and drift, but insufficient accuracy for the determination of vessel speed and leeway. The GPS determines only position, and vessel speed must be calculated by taking the difference between two position readings which gives the distance the vessel has moved, and dividing that difference by the time between the position readings. The problem is that if a readout of speed is to be used for sail adjustment, the readout must be consistent and have a reasonable rate of response. The GPS cannot simultaneously meet these requirements satisfactorily.
For example, if a vessel is moving at a speed of five knots in the true north direction, and the time between readouts is two seconds, which is common for a microprocessor-based system, the vessel moves 3.38 feet in those two seconds, which corresponds to a change in position of 0.00056 minutes of latitude. But, if an inconsistency of .+-.20 feet is assumed for the GPS reading, a meaningful vessel speed cannot be obtained therefrom at a response rate of two seconds. In fact, assuming a consistency of 0.1 knots is required, the time between position readings used in the speed calculation would have to be approximately 23.7 sec. That rate of response is far too slow for the purposes of sail trimming and determining the best angle relative to the wind for optimum operation of the sailboat.
Another consideration is that speed measurement accuracy or consistency is speed dependent. For a sailboat, a speed of much less than 5 knots is often encountered. Accordingly, it is necessary to measure speed of the vessel directly using one of the means already available for that purpose.
The meaning of the term "consistency" as used herein will now be explained. In a digital readout, if the quantity displayed varies, not because the quantity being measured actually varies but because of the errors in the measurement of that quantity, the digital display will vary around the actual value. For example, even if the vessel is proceeding at a constant speed of 5.0, errors in the measurement of speed by position difference as determined by a satellite system may give a series of readings at two second intervals such as 5.0, 4.9, 5.2, 4.8, 5.0, etc. Such scattered measurements are not very useful to those attempting to trim the sails in order to achieve maximum speed of the sailboat.
The measurement of leeway is subject to the same problems as is the measurement of speed, and even at a greater extent because the rate of movement in the cross-track direction is normally much less than the speed in the direction parallel to the center line of the craft. Because leeway must be known to determine the speed and direction of the vessel relative to the bottom, which in turn must be known for the proper steering of the vessel, leeway must also be measured directly and not by the GPS.
The determination of current set and drift is different than for vessel speed and leeway because the two former variables change relatively slowly except in exceptional circumstances. Thus, an appropriate amount of vessel movement can be allowed between the two position readings used to establish set and drift to provide the required level of accuracy. An accuracy of .+-.20 feet is useless for the measurement of speed, as well as for current set and drift, if the vessel moves only 10 feet between readings. But if the vessel were to move 1000 feet between readings, these factors could be determined with an accuracy of about 2%, which represents a substantial improvement over any other presently available system.
The Mounce microprocessor-based system determines vessel position by an integration of the data measured by its sensors. Because these individual data have a measurement accuracy factor, while the percentage accuracy in the determination of distance and direction from a starting point remains relatively constant, the absolute positional accuracy gradually deteriorates. A position determining system such as GPS provides a relatively constant and high accuracy position determination. Using the position determined by the GPS or a like system, the position determined by the Mounce microprocessor-based system could be updated such that its accuracy becomes substantially equivalent to that of the GPS, while it still maintains the rate of response necessary for the moment to moment control of the vessel. Thus, the integration of the two systems would result in an improved system having the best features of each.
With respect to the sailing direction to reach the destination, both the GPS and the Mounce microprocessor-based system are capable of determining it with approximately equal accuracy, but it is more convenient to have this direction displayed on the readout associated with the microprocessor aboard the vessel.
With respect to the measurement and display of wind direction and speed, the GPS is unable to determine either value. Consequently, these values must be determined onboard by wind speed and direction sensors, and the microprocessor-based system can display them relative to the vessel and also calculate and display the direction and speed of the wind relative to the earth and to the vessel. A number of systems are available for measurement and display of wind speed and direction relative to the vessel because that information is supplied directly by sensors. The determination of the actual wind direction and speed relative to the earth requires a vector resolution using the values calculated for the speed and direction of the vessel. These values can only be provided by a microprocessor-based system.
There has also been a need in the art for a navigational aid system that includes a sensor for measuring the angle of heel of the vessel. A heel angle sensor would provide a readout of the angle by which the aspect of the vessel in a direction at right angles to the center line deviates from the aspect when the vessel is at rest and not acted upon by wind or current. The heel angle can be important during the operation of a vessel, not only as an indicator of when the vessel is approaching an unsafe degree of heel or tilt, but also as an aid to maximizing vessel speed. This is because a given vessel has a specific range of values for heel in a given set of sailing conditions over which the greatest speed through the water can be attained.
In addition to making it possible to display the heel angle, such a sensor would improve the accuracy of the value of leeway as determined by the microprocessor. The known method of measuring the leeway angle is by a vane mounted on the underside of the vessel and attached to an electromechanical assembly which produces a signal proportional to the angle of the vane relative to the center line of the vessel. For a given set of conditions, the leeway angle thus measured is reduced as the vessel heels.
The Mounce system includes a display consisting of a first alpha display for designating a quantity that is indicated by a second display adjacent to the alpha display. The Mounce system is inadequate, however, because a continuous readout of the actual boat direction with respect to the earth is impossible if other parameters are displayed. This quantity should be available to the helmsman at all times so that the vessel can be steered in the desired course. Thus, there has been a need for a display that provides this information.
There has also been a need for a pair of onboard displays; one providing an alpha and numeric readout of the actual boat direction with respect to the earth, and the other providing the same two types of readouts for a manually selectable quantity.